Surface Tension  

Amina Grant                    Shakespeare
l448 East 52nd. St. #252       1119 East 46th St.
Chicago IL 60615               Chicago IL 60653
312-324-2229                   312-535-1560

Objective: 

     Students will observe forces between the water molecules which form
Surface Tension.

Materials:

     The following materials are for a class:
          1. overhead
          2. beaker
          3. straight pins
          4. water
          5. wire (24 cm. long to make a cradle for straight pin)
          6. medicine-cups (to hold 1 part Joy)
          7. water soluable pens
          8. liquid detergent (Joy) & Glycerin
          9. square wire frame (use the same wire from cradle)

Strategies:
     
                 Surface Tension

          a.  Fill the beaker with water, (each student has his own beaker and 
straight pin).
          b.  Take the straight pin and show that it is not possible to float a
straight pin on the water when it is dropped vertically into the water.
          c.  Fish straight pin out of the water and dry it off or use another 
needle.
          d.  Now hold the needle horizontally as close as possible to the 
surface of the water (without touching the water) and drop it.
                           
                 Questions

      1.  Why is it not possible to float the needle on the water when it 
is dropped vertically?
      2.  Why is it not possible to float the needle on the water when it
is dropped horizontally?
      3.  What happened?
      4.  Why did it happen?
         
          e.  The teacher will say, "I predict the straight pin will sit on top 
of the water if placed right."
          f.  Give students a piece of wire, demonstrate how to make a "wire 
cradle" for the straight pin.
          g.  Put the straight pin on the cradle lift it to the surface of the
water, remove the cradle without pulling straight pin beneath the surface.

                 Question

      1.  What property of the water keeps the straight pin afloat?

     EXPLANATION:

          When the needle is dropped vertically into the water, it pierces the 
water surface.  The surface tension at that very small area of the straight pin 
is not large enough to hold the needle afloat.  But when the needle is dropped 
horizontally, a much larger area of the water surface is involved.  When the 
cradle serves to balance the straight pin on the water surface the forces 
between the water molecules under and around the straight pin are large enough 
to hold the needle afloat. 

II.    At this point include a brief talk about molecules.  Use a transparency 
and overhead to illustrate the forces at work between the molecules.

III.   The Circle Inside The Frame

          a.  Make a soap solution by mixing the following liquids: 1 part Joy, 
21/2 parts glycerin, and 3 parts water.
          b.  Place this soap solution in the shallow container, make a soap 
film by dipping the wire frame in the solution and taking it slowly out.
          c.  Make a small loop of thread (diam. 3-4cm), wet it in the soap
solution and lay it carefully in the soap film.
          d.  Once the thread loop lies in the soap film, pierce the center of 
the loop with a dry object (pencil or dry finger).
          e.  Slant the wire frame, wiggle it, and observe the perfect thread
circle move and travel throughout the whole frame.

                 Question(s)

     1.  What made the wrinkley thread turn into a perfect circle?
     2.  What does this tell us about the forces working on it in the soap
film?

     EXPLANATION:

          This demonstration shows that there is indeed a tension working in 
the soap film.  However, the surface tension of water is much greater than that 
of soapy water.

Performance Assessment:

          The performance assessment that I would use represents COLLABORATIVE
          LEARNING, teacher and students evaluate an investigation.
          i.e. 1.  What did we do that helped you most to find an answer?
               2.  What could we have done better?
               3.  What new ideas did we discover?
               4.  What materials and equipment helped us most?
               5.  How did we use measurements to help find answers?

    Scoring Rubric:

               1.  5 points:  an explanation that is complete
               2.  4 points:  if student includes a diagram (a)
               3.  3 points:  the response is generally correct, but the 
                              explanation lacks clarity.
               4.  2 points:  an object identification, more than one.
               5.  1 points:  the response is incorrect, wrong use of term(s) 

                  
Conclusions:

          Each project will be judged according to its relationship to the 
     NGUZA SABA principles of Blackness.  
     e.g. PRINCIPLES     

          1. Umoja (unity) Does project encompass at least one science area?
          2. Kujichagulia (self-determination) Does the project demonstrate an 
             understanding of a need for Afrikan people to develop solutions for
             their particular needs?
          3. Ujima (collective work and responsibility) Is there evidence of 
             shared responsibility for the development and care of the exhibit?
          4. Ujamaa (cooperative economics) Were the materials for the project
             obtained using self-reliant means?
          5. Nia (purpose) Is the project based on a hypothesis?
          6. Kuumba (creativity) Are students able to discuss project without
             reciting the written material of the exhibit?
          7. Imani (faith) Is there evidence of dedication among the scientists?

References:

     William A. Andrews, T. J. Elgin Wolfe & John F. Eix, Physical Science, 
(Prentice-Hall, Canada,) 1998.
     Paul G. Hewitt, Conceptual Physics 6th edition, (Scott, Foresman,) 1989
     Earl Zwicker, 1992
     Tiek Liem